As compared to metal racks, racks made of a thermoplastic resin have excellent flexibility and are used more advantageously for certain purposes. Thus, racks of thermoplastic resin have recently been used in many end-use applications.
Most conventional racks of thermoplastic resin are obtained by injection molding or by cutting and shaping performed extrusion molded rods. However, manufacturing a rack by cutting a preformed rod of a thermoplastic resin is a troublesome process since the valleys of the teeth of the rack are formed one by one, and, therefore the manufacturing costs are high. On the other hand, when racks of thermoplastic resin are made by injection molding, it is difficult to obtain a product having large length due to inherent limitations of the manufacturing process for example the limitation on the size of a metal mold that may used. Moreover, since the mold is mold of metal uneven thermal contraction of the resin necessarily occurs. As a result it is very difficult to obtain a rack having uniform properties and dimensional accuracy. A thermoplastic resin is most often used in place where a flexible rack is required. Flexible racks must be sufficiently so as to resist bending tough when subjected to significant deformation. Racks obtained by cutting a performed extrusion molded rod back along a surface flaw which occurs during the cutting operation when deformed. Similarly, injection molded racks break at a portion thereof which has uneven properties.
Japanese Patent Laid-Open No. 63418/1983 (hereinafter "the Japanese '418 publication") proposes a method of manufacturing racks by profile extrusion molding, i.e. a method of forming teeth on an extruded resin not yet completely solidified. According to this method, the above-mentioned problems are solved to a considerable extent. However, special equipment provided with an extruder is required. Moreover, in order to obtain a product having a predetermined dimensional accuracy, it is necessary that an extrusion step and a rack-forming step be matched accurately with one another. Since a significant amount of time is needed in order to set the conditions for performing these steps properly, the method disclosed in the Japanese '418 publication is not suitable for small quantity productions of multiple rack types.
According to the rack manufacturing method disclosed in the Japanese '418 publication teeth for the rack are formed in a resin in a molten state, with the resultant product thereafter being cooled. Therefore, voids tend to be present in the rack. The method disclosed in the Japanese '418 publication also does not completely meet the requirements for obtaining a highly durable rack free from internal defects.
The present invention is therefore directed to method and apparatus for obtaining a thermoplastic resin rack having high accuracy, durability and toughness. More specifically, the present invention is directed to method and apparatus whereby a rod of a thermoplastic resin is subjected to hot plastic forming under special conditions.
Namely, this invention relates to method and apparatus for manufacturing racks of a thermoplastic resin, in which a rod of a thermoplastic resin is heated (e.g. with microwave energy so that the temperature of at least the outer surface thereof reaches a level expressed by the equation, EQU T-20.degree. C..ltoreq.Ts&lt;T (1)
(wherein T is a melting point of a resin forming a matrix for the rod and Ts is the surface temperature of the rod. A rack is thereafter formed on one or both surfaces of the resultant rod by means of a toothed roller and a grooved roller which are paired with one another or by means of two opposed toothed rollers respectively.